This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The way in which cells achieve their differentiation program is intimately tied to changes in their proteome occurring via several mechanisms including, transcription factor activation or inhibition of transcription events, tissue specific regulation of splicing machinery resulting in alternatively spliced transcripts, ubiquitin dependant proteosomal degradation, and post-translational modification. Determining the molecular mechanisms involved in regulating changes in the proteome of differentiating cells, is proving to be crucial for understanding and controlling tissue regeneration and extremely valuable for the continued improvement of stem cell therapies and proteomic based clinical markers of disease. Several key observations have led to the hypothesis that Ufd2a may be one regulator of this process during differentiation of striated muscle tissue. Previous data has shown that undifferentiated mammalian myoblasts express exclusively a shorter, ubiquitous form of Ufd2a. However, upon initiation of the differentiation program both in vitro and in vivo, these myoblast begin to express alternatively spliced isoforms of Ufd2a which include two unique exons. In addition, Nakayama et al has shown that Ufd2-/- mice die in utero with multiple heart defects, implying that Ufd2a is critical to normal cardiac development. Finally, given that one of the four key steps of muscle cell differentiation involves cell cycle exit, it is of interest that Ufd2a was shown to be critical to cell division and might also participate in apoptosis signaling. This proposal addresses the hypothesis that the alternative isoforms of Ufd2a are important for the process of striated muscle cell differentiation and development. These studies may provide insights into how alternative splicing can regulate critical cell cycle effectors in non-dividing cells, and lead to a better understanding of the mechanisms of cardiac and skeletal muscle development. The proposed experiments will address the following specific aims: Specific aim 1: Validate the use of Zebrafish as a model for studying the significance of Ufd2a to muscle development. Specific aim 2: Define the significance of novel Ufd2a isoforms to the process of muscle differentiation in C2C12 cells. Specific aim 3: Define the catalytic activity of the novel Ufd2a isoforms.